Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
U.S. Government and agency obligations
Investment securities
Cost or Amortized Cost	$ 209.0	$ 216.6
Gross Unrealized Gains	0.3	0.0
Gross Unrealized Losses	(4.4)	(10.2)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	204.9	206.4
Debt securities issued by corporations
Investment securities
Cost or Amortized Cost	1,085.9	1,098.3
Gross Unrealized Gains	5.6	0.6
Gross Unrealized Losses	(45.4)	(78.3)
Net Foreign Currency Gains (Losses)	(1.1)	(1.8)
Carrying Value	1,045.0	1,018.8
Municipal obligations
Investment securities
Cost or Amortized Cost	275.1	281.6
Gross Unrealized Gains	0.9	0.4
Gross Unrealized Losses	(15.1)	(23.4)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	260.9	258.6
Mortgage and asset-backed securities
Investment securities
Cost or Amortized Cost	417.2	288.7
Gross Unrealized Gains	2.0	0.0
Gross Unrealized Losses	(29.8)	(34.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	389.4	254.2
Collateralized loan obligations
Investment securities
Cost or Amortized Cost	211.2	190.8
Gross Unrealized Gains	0.4	0.1
Gross Unrealized Losses	(2.4)	(6.0)
Net Foreign Currency Gains (Losses)	(0.1)	(2.0)
Carrying Value	209.1	182.9
Fixed maturity investments
Investment securities
Cost or Amortized Cost	2,198.4	2,076.0
Gross Unrealized Gains	9.2	1.1
Gross Unrealized Losses	(97.1)	(152.4)
Net Foreign Currency Gains (Losses)	(1.2)	(3.8)
Carrying Value	$ 2,109.3	$ 1,920.9